The present invention relates to a process for the electro-chemical treatment of hardened concrete in order to modify (i.e. by increasing or decreasing) the bond strength between the hardened concrete and internally embedded steel, particularly reinforcing bars, pre-tensioning or post-tensioning rods or cables. Heretofore, this has been impossible, since there has been no known procedure for controllably changing the steel-to-concrete bond, in situ, in hardened concrete.
An additional aspect of the invention involves the modification of the steel-to-concrete interface in a hardened concrete structure to enhance the seal at such interface. Frequently, the interface seal between embedded steel reinforcing or tensioning elements is less than perfect, due to accumulation of bleeding water at the steel surface during the initial hardening of the concrete, or possibly due to insufficient compaction of the concrete when initially poured. Imperfections in the seal at the steel-to-concrete interface can result in seepages, in structures exposed to water pressure, or a possible carbonation of the concrete surfaces adjacent to the steel, with consequent corrosion of the steel.
The present invention is based in part upon the discovery that, during the electro-chemical treatment of concrete, utilizing the internally embedded steel as a cathode, and a distributed electrode structure spaced therefrom, typically at an exposed surface of the concrete, as an anode, a marked and permanent change occurs in the bond between the embedded steel and the surrounding concrete, as a function of the electrical charge applied. During an initial phase of the treatment, there is a progressive and significant reduction in the bond strength to a level far below the initial bond strength. This is followed, with continued treatment, by a progressive and significant increase in bond strength. I have observed that this variation in bond strength is both predictable and repeatable for given types of concrete. Accordingly, by establishing a simple database of relationships between a given treatment time and its effect upon the steel-to-concrete bond strength, it becomes possible to predictably modify such bond strength in an existing structure.
In the case of pre-tensioned or post-tensioned concrete structures, for example, it may be desirable to decrease the bond strength at the steel-to-concrete interface. This would tend to better accommodate flexing of the compressed concrete structural element. With static steel reinforcing bars, on the other hand, it may be desirable to effect an increased bond at the steel-to-concrete interface.
There are known electro-chemical treatments for reinforced concrete, for example, in which the internal steel is connected as a cathode and an external distributed electrode is used as an anode. An example, of such is the Vennesland et al. U.S. Pat. No. 4,832,803. Such known procedures, however, are intended for use in the rehabilitation of concrete structures which have become contaminated with chlorides, for example, or where conditions have become corrosive to the internally embedded steel, through carbonation or otherwise. While the techniques of the invention might still be usefully employed even in connection with contaminated concrete, the treatment of the invention is not a rehabilitative process, but is directed to controlling and modifying the bond at the steel-to-concrete interface. In the process of the present invention, treatment conditions and controlling parameters are altogether different than for rehabilitation.
For a more complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of preferred practices of the invention, and to the accompanying drawings.